1. Field of the Invention
The present invention relates to an optical disk apparatus having a mechanism for positioning an optical spot on an optical disk, and particularly to an optical disk apparatus having a separated optical system in which an objective lens and a focusing mechanism only are moved for access.
2. Description of Related Art
Although conventional optical disk apparatus having a separated optical system include various types, an optical disk apparatus of the type in which an objective lens and a focusing mechanism are mounted on an access mechanism is most light weight. An example of this type is the optical disk apparatus described in Reports of the Spring National Meeting of the Society of Electronic Information Communication in 1990, Volume 4, p. 474. In this apparatus, a galvanomirror is provided in a fixed optical system, and an optical spot is roughly positioned in the radial direction of a disk by moving an objective lens mounted on an access mechanism and is highly precisely positioned by rotating the galvanomirror through an angle corresponding to a differential amplitude of a high-frequency wave. There is no problem when the galvanomirror and the objective lens are placed at a short distance. However, when the objective lens is moved toward the inner periphery of the disk to increase a distance between the galvanomirror and the objective lens, if the galvanomirror is inclined from a neutral position (reference position), the following two problems are raised. A first problem is that, since the flux of light incident upon an objective lens is shifted from the optical axis of the objective lens, the diameter of the flux must be sufficiently increased so as to prevent the flux of light incident upon the objective lens from missing (changing in the sectional area of the flux). The intensity of the flux of light incident upon the objective lens is thus decreased. A second problem is that the position of the flux of light reflected from the disk and returned to the fixed optical system is deviated from the position of the flux of light incident upon the objective lens. In a differential diffraction system which is widely used as a track detecting system for detecting a relative positional deviation between an optical spot and a recording track on a disk, the above-described positional deviation of the flux of light returned to the fixed optical system produces an offset of a detection signal of the positional deviation between the optical spot and the recording track. A method has been thus proposed for decreasing the positional deviation of the flux of returned light, which is caused when the distance between the galvanomirror and the objective lens is increased. This method is disclosed in Japanese Patent Publication No. 58-6211. According to this publication, the rotational center of a galvanomirror is deviated from the center of the galvanomirror in correspondence with the distance between the center of the galvanomirror and the objective lens. However, this method has the problems that the response of the galvanomirror is deteriorated, and it is impossible to follow a positional deviation of a high frequency wave. If the distance between the rotational center and the center of the galvanomirror is increased due to an increase in the distance between the objective lens and the galvanomirror, the rigidity of a galvanomirror actuator is decreased and a moment of inertia is increased. These are the reasons why such problems are occurred.
Although the conventional optical disk apparatuses have the effect of decreasing a positional deviation of the optical axis of the flux of light reflected and returned from the disk, the disk apparatuses have the problem that high response required for a galvanomirror cannot be realized.